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REGULATORY MATTERS (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Total Capital (to risk-weighted assets) [Abstract]    
Actual capital ratios, Amount $ 209,314us-gaap_Capital $ 201,476us-gaap_Capital
Minimum for capital adequacy, Amount 122,866us-gaap_CapitalRequiredForCapitalAdequacy 121,608us-gaap_CapitalRequiredForCapitalAdequacy
Minimum to be Well Capitalized under prompt corrective action provisions, Amount 153,582us-gaap_CapitalRequiredToBeWellCapitalized 152,010us-gaap_CapitalRequiredToBeWellCapitalized
Total Capital (to risk-weighted assets) Ratios [Abstract]    
Actual capital ratios, Ratio (in hundredths) 13.63%us-gaap_CapitalToRiskWeightedAssets 13.25%us-gaap_CapitalToRiskWeightedAssets
Minimum for capital adequacy, Ratio (in hundredths) 8.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets 8.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Minimum to be Well Capitalized under prompt corrective action provisions, Ratio (in hundredths) 10.00%us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets 10.00%us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Tier 1 Capital (to risk-weighted assets) [Abstract]    
Actual capital ratios, Amount 190,111us-gaap_TierOneRiskBasedCapital 182,469us-gaap_TierOneRiskBasedCapital
Minimum for capital adequacy, Amount 92,149us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy 60,804us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy
Minimum to be Well Capitalized under prompt corrective action provisions, Amount 122,866us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized 91,206us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized
Tier 1 Capital (to risk-weighted assets) Ratio [Abstract]    
Actual capital ratios, ratio (in hundredths) 12.38%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets 12.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
Minimum for capital adequacy, Ratio (in hundredths) 6.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets 4.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Minimum to be Well Capitalized under prompt corrective action provisions, Ratio (in hundredths) 8.00%us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets 6.00%us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Common Equity Tier 1 Capital (to risk-weighted assets) [Abstract]    
Actual capital ratios, Amount 190,111subk_CommonEquityTierOneRiskBasedCapital  
Minimum for capital adequacy, Amount 69,112subk_CommonEquityTierOneRiskBasedCapitalRequiredForCapitalAdequacy  
Minimum to be Well Capitalized under prompt corrective action provisions, Amount 99,829subk_CommonEquityTierOneRiskBasedCapitalRequiredToBeWellCapitalized  
Common Equity Tier 1 Capital (to risk-weighted assets) Ratios [Abstract]    
Actual capital ratios, Ratio (in hundredths) 12.38%subk_CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssets  
Minimum for capital adequacy, Ratio (in hundredths) 4.50%subk_CommonEquityTierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets  
Minimum to be Well Capitalized under prompt corrective action provisions, Ratio (in hundredths) 6.50%subk_CommonEquityTierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets  
Tier 1 Capital (to average assets) [Abstract]    
Actual capital ratios, Amount 190,111us-gaap_TierOneLeverageCapital 182,469us-gaap_TierOneLeverageCapital
Minimum for capital adequacy, Amount 75,915us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy 73,312us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy
Minimum to be Well Capitalized under prompt corrective action provisions, Amount $ 94,894us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized $ 91,640us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized
Tier 1 Capital (to average assets) Ratios [Abstract]    
Actual capital ratios, ratio (in hundredths) 10.02%us-gaap_TierOneLeverageCapitalToAverageAssets 9.96%us-gaap_TierOneLeverageCapitalToAverageAssets
Minimum for capital adequacy, Ratio (in hundredths) 4.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets 4.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets
Minimum to be Well Capitalized under prompt corrective action provisions, Ratio (in hundredths) 5.00%us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets 5.00%us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets
Tier 1 Leverage Capital ratio (in hundredths) 10.02%us-gaap_TierOneLeverageCapitalToAverageAssets 9.96%us-gaap_TierOneLeverageCapitalToAverageAssets
Tier 1 Risk-based Capital Ratio (in hundredths) 12.38%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets 12.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
Common Equity Tier 1 Risk-based Capital Ratio (in hundredths) 12.38%subk_CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssets  
Total Risk-based Capital Ratio (in hundredths) 13.63%us-gaap_CapitalToRiskWeightedAssets 13.25%us-gaap_CapitalToRiskWeightedAssets
Parent Company [Member]    
Total Capital (to risk-weighted assets) Ratios [Abstract]    
Actual capital ratios, Ratio (in hundredths) 13.77%us-gaap_CapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
13.35%us-gaap_CapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Tier 1 Capital (to risk-weighted assets) Ratio [Abstract]    
Actual capital ratios, ratio (in hundredths) 12.52%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
12.10%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Common Equity Tier 1 Capital (to risk-weighted assets) Ratios [Abstract]    
Actual capital ratios, Ratio (in hundredths) 12.52%subk_CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
 
Tier 1 Capital (to average assets) Ratios [Abstract]    
Actual capital ratios, ratio (in hundredths) 10.13%us-gaap_TierOneLeverageCapitalToAverageAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
10.04%us-gaap_TierOneLeverageCapitalToAverageAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Tier 1 Leverage Capital ratio (in hundredths) 10.13%us-gaap_TierOneLeverageCapitalToAverageAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
10.04%us-gaap_TierOneLeverageCapitalToAverageAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Tier 1 Risk-based Capital Ratio (in hundredths) 12.52%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
12.10%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Common Equity Tier 1 Risk-based Capital Ratio (in hundredths) 12.52%subk_CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
 
Total Risk-based Capital Ratio (in hundredths) 13.77%us-gaap_CapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
13.35%us-gaap_CapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember